904 slipping from 2nd-3rd (with video)

A&A transmission have been real great to work with for me. aandatrans.com
I have thier parts in my 904 including manual valve body and it has worked great for 6 years. Pulled it down for freshening and everything inside was great.
If you pull that trans out for repairs, make sure the clutch clearances are within spec and endplay as well. The return spring on the front clutch should be updated to the multi spring.
I would recommend a manual valve body for your application.